1. Field of the Invention
The present invention relates to a display device, and in particular, to methods for producing a switching element used for the display device and for producing the display device.
2. Description of the Related Art
In recent years, significant progress has been made in the use of low-profile display devices such as a liquid crystal display and an organic EL (electro luminescence) display device. Of the low-profile display devices (FPD), an active matrix type display device capable of high-quality display in particular has three-terminal elements such as an amorphous silicon TFT (thin-film transistor), a polysilicon TFT and two-terminal elements such as an MIM (Metal-Insulator-Metal) used as switching elements thereon.
These switching elements are formed on a substrate constituting a display device in a process of manufacturing the display device. The switching elements are generally formed by repeating a film deposition process and a patterning process a plurality of times. For instance, in the case of a TFT, it is necessary to repeat the film deposition process and patterning process at least three times, and these processes are usually repeated four to eight times. In the case of the MIM, it is necessary to repeat them at least twice, and they are usually repeated three to four times.
The switching element is formed in each pixel. Because the area of the switching element is smaller than the area of the pixel, various thin films (a metallic layer, an insulating layer and a semiconductor layer for instance) which are deposited on almost the entire surface of the substrate are mostly removed in the patterning process. For instance, in the case of a transmissive liquid crystal display, approximately 90 percent of the metallic layer and semiconductor layer is removed in order to increase a pixel open area ratio.
The patterning process is mostly performed by a photolithographic process using photomasks, where it is necessary to align the photomasks with high precision in each process in order to suppress variations in element characteristics.
Furthermore, the substrates are becoming larger in size due to increasing size of display devices or for the sake of improving manufacturing efficiency, and so an apparatus for performing the film deposition process and patterning process is becoming larger in size.
As described above, the active matrix type FPD in the past has most of the materials for forming the switching element removed so that, as resources and energy are lost, costs thereof increase and a load to the environment also increases.
In addition, for the purposes of coping with the larger-size substrates, an expensive film deposition apparatus and a photolithographic apparatus for forming the switching element become larger in size. For that reason, manufacturing costs increase, and it becomes difficult to form the switching element having little variations in element characteristics over a large area.
The present invention has been implemented in consideration of the above points, and a primary object thereof is to provide an active matrix type display device which can be manufactured by an easier method than before and a manufacturing method of such a display device. Another object is to provide a manufacturing method of a bidirectional two-terminal element suitably used as a switching element of such a display device.